Uterine cancer arises from the lining of the uterus (the womb) and is the most commonly diagnosed gynecologic malignancy in the United States. Uterine tumors are classified into at least 10 different subtypes. One of these subtypes is uterine carcinosarcoma, also known as metaplastic carcinoma or as malignant mixed Mllerian tumor of the uterus. This histological subtype is rare at diagnosis and is generally associated with a very unfavorable prognosis. In previous reporting periods we applied whole exome sequencing and targeted gene sequencing to search for somatic mutations that drive, or potentially drive, the development of uterine carcinosarcomas (Le Gallo et al., Cancer 2017). The most frequently mutated genes uncovered in our analyses included PIK3CA, PPP2R1A, FBXW7, CHD4, and FOXA2. Our study was the first to identify FOXA2 somatic mutations in uterine carcinosarcomas. The frequent occurrence (15%) of FOXA2 mutations among carcinosarcomas coupled with the predominance of frameshift and nonsense mutations strongly suggested that FOXA2 mutations are pathogenic driver events in a subset of uterine carcinosarcomas. At the end of the last reporting period we initiated a new project to investigate the functional consequences of RIT1 alterations in uterine carcinosarcoma. In The Cancer Genome Atlas (TCGA), somatic copy number alterations are common in uterine carcinosarcomas and include a chromosome 1q22 amplification encompassing the RIT1 gene. A detailed query of TCGAs uterine cancer datasets shows that RIT1 is amplified in 14% of uterine carcinosarcomas, 22% of serous endometrial carcinomas (ECs), and 4% of endometrioid ECs. By extending the query to include somatic mutations and dysregulated gene expression, the incidence of RIT1 genomic alterations increases to 39% in uterine carcinosarcoma, 26% in serous EC, and 27% in endometrioid EC, with most of the additional cases being attributed to mRNA upregulation; in uterine carcinosarcomas RIT1 amplification was almost always accompanied by RIT1 mRNA upregulation. The RIT1 (Ras like without CAAX-1) small GTPase is a component of the RAS-MAPK pathway. Pathogenic germline variants in RIT1 account for about 5% of Noonan Syndrome cases, one of several so-called RASopathies. Evidence supporting RIT1 as a cancer gene comes primarily from studies in lung cancer in which oncogenic (gain-of-function) RIT1 mutations have been found in a subset of tumors. Here, we hypothesize that amplification and/or overexpression of the RIT1 gene is oncogenic in uterine carcinosarcoma. In ongoing studies, and future studies that will extend into the next reporting period, we are conducting biochemical and cell-based experiments to test this hypothesis.